1. Field of the Invention
The invention relates to nano-structured powders, and in particular to powders with superhydrophobic and self-cleaning functions and a fabrication method thereof.
2. Description of the Related Art
Superhydrophobicity is exhibited by the unique microstructure and chemical properties of the leaves of certain plants in nature. The characteristic of a superhydrophobic surface is that water forms spherical droplets immediately when dropping on the surface. Such a superhydrophobic surface possesses a so-called “self-cleaning” function. For example, droplets formed on a lotus leaf carry dust away after rolling off the leaf. This superhydrophobic phenomenon is caused by the roughness and low surface tension of the surface.
When a liquid drops on a surface of an object, an angle is then formed between the surface and the tangent line of the drop, called contact angle θ. When the tension in the gas-solid interface (also called solid surface energy) is high, the contact angle tends to be low, which means the surface easily gets wet. Contact angle of 0 degree indicates that water spreads over and becomes a film on the surface so that the surface would completely get wet. On the contrary, if the tension in the gas-solid interface is low, the contact angle tends to be high, indicating that the surface does not easily get wet. Therefore, contact angle of 180 degrees means that the liquid forms spherical droplets and cannot wet the surface at all.
When the surface of the object is rougher, the contact angle would normally become higher. This is because surface cavities with high aspect ratio may trap air so that the droplet sits on a composite surface made of air and solid.
Presently, there are many methods for preparing a water-repellent surface. For example, U.S. Pat. No. 5,693,236 provides a water-repellent structure and its fabrication method including the steps of: preparing a mixture by mixing a curable liquid with a needle-like material; applying the mixture onto a surface of an object; curing a liquid of the applied mixture and forming an applied layer in which the needle-like material has been mixed on a base material of the cured liquid of the mixture; forming pits and projections of the needle-like material on a surface of the applied layer by etching the applied layer under a condition where an etching rate of the base material is larger than that of the needle-like material; and coating the surface of the applied layer with a water-repellent substance.
The self-cleaning properties are arousing much interest because of their water and dirt repellency. The self-cleaning effect can be applied to building materials such as glass, woods, rocks, or ceramic tiles as well as cars and plastics. Self-cleaning coatings not only save us cleaning and other maintenance costs, but also are good for ecobalance since use of water and cleaning chemicals as well as maintenance frequencies can be greatly reduced or eliminated. Although traditional paints can provide a water-proof coating, droplets formed thereon cannot effectively roll off to carry dust away, thus providing no self-cleaning functions. Therefore, it is desirable to develop a superhydrophobic coating with low surface energy as well as proper surface roughness in order to possess self-cleaning functions.